Laser beams have an intensive energy, and have heretofore been used to increase the temperature of a surface of a material to which the laser beam is applied thereby to ablate or evaporate a portion of the material to which the laser beam is applied, for processing the material in various ways. Since the laser beam can be focused into a very small spot, it is suitable for microscopic topographic processing of a material.
Laser beams are generated by an infrared laser such as a CO.sub.2 laser, an Nd:YAG laser, a laser comprising an Nd:YAG laser combined with a wavelength conversion capability for producing a laser beam whose wavelength ranges from a near-infrared region through a visible region to an ultraviolet region, and an ultraviolet laser such as an excimer laser such as an Arf or Krf laser.
Silicate glass composed primarily of SiO.sub.2 is highly transparent and can easily be molded (deformed) at high temperatures. Sheets of silicate glass, which have been formed with holes or concavities and convexities by microscopic topographic processing, are widely used as glass substrates for optical components used for optical communications and display devices.
In order to make a hole in a sheet of silicate glass according to microscopic topographic processing, it has been the general practice to process the sheet of silicate glass with wet etching (chemical etching) using an etchant of hydrofluoric acid or the like, or dry etching (physical etching) such as reactive ion etching.
However, the wet etching suffers problems with respect to management and processing of the etchant. The dry etching requires pieces of equipment such as a vacuum container, needs a large-scale apparatus, and is not efficient because a pattern mask has to be produced by complex photolithography.
It has been attempted to use a laser beam for microscopic topographic processing of glass. Since glass is fragile, it tends to crack when processed. If an infrared carbon dioxide laser is used to process glass, the glass will violently crack due to thermal strain.
If an ultraviolet KrF excimer laser (wavelength of 248 nm) is used to process glass, the glass will crack around the area where the laser beam is applied. The ultraviolet KrF excimer laser (wavelength of 248 nm) is thus not suitable for microscopic topographic processing of glass. The use of an ArF excimer laser having a wavelength of 193 nm for emitting a laser beam to process glass is optimum. However, even when such an ArF excimer laser is used, the generation of microcracks is unavoidable.
The ArF excimer laser having a wavelength of 193 nm is subject to absorption by air, and needs to replace the transmission medium with an absorption-free gas such as Ar or a vacuum in order to allow the laser beam to reach as far away as possible.
There has been proposed a technique disclosed in Japanese laid-open patent publication No.54-28590. According to the disclosed technique, when a laser beam is applied to process a glass substrate, the glass substrate has been heated to 300.about.700.degree. C. in advance to withstand thermal shocks caused when it is processed with the laser beam.
When the glass substrate is subject to microscopic topographic processing by the laser beam while it is being heated to relax stresses, however, the glass substrate cannot be processed to an accuracy ranging from micrometers to submicrometers because of thermal shrinkage.
Even when the glass substrate is subject to microscopic topographic processing by the laser beam while it is being heated, the processed area has a rough surface, but not a smooth finish. The processed glass substrate is still susceptible to cracking or breakage.
The inventors have attempted to apply a laser beam to general photosensitive glass which contains a uniform concentration of Ag ions. The process of applying the laser beam to the general photosensitive glass will be described below with reference to FIGS. 1(a) 1(d) of the accompanying drawings. As shown in FIG. 1 (a), a laser beam applied to a glass substrate enters into the glass substrate, reducing Ag ions present in the glass substrate as shown in FIG. 1(b) thereby to generate a colloid (very fine particles of Ag). When the colloid is separated out, as shown in FIG. 1(c), the coefficient of absorption of the laser beam is greatly increased. The glass substrate now starts being ablated from inside thereof until finally it develops a recess-like crack or breakage as shown in FIG. 1(d).